Steam blowing assembly for nuclear power plants

ABSTRACT

The invention relates to a steam blowing tube for nuclear power plants of the kind which has a lower end immersed in a water pool, the purpose of tube being to permit, if required, the blow down of steam into the water of the pool with the purpose of condensing the same so as to counteract the build-up of damaging steam overpressure in the containment of the reactor. A number of through holes are recessed in the wall (11) of the individual blow down tube (10) which each one is delimited by upper and lower edges (13, 14) of which at least one constitutes part of a portion (15) bent in relation to the tube wall (14). The lower hole edge (14) forms a steam flow cutting edge which, when the steam rushes down through the tube, divides a part flow from the main steam flow and brings said part flow out into the surrounding water of the pool before the main flow reaches the bottom opening (12) of the tube.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a steam blowing assembly for nuclear powerplants of the kind which comprises a plurality of tubes, open at the topand at the bottom, hanging in connection with a reactor, the lower endsof which tubes are immersed in a water pool and the purpose of which isto permit, if required, the blow down of steam into the water of thepool with the purpose of condensing the same so as to counteract thebuild-up of damaging steam overpressure in the containment of thereactor.

BACKGROUND OF THE INVENTION AND PRIOR ART

An assembly of the above-mentioned kind is schematically illustrated inFIG. 1, where 1 generally designates a containment for a nuclear powerreactor 2. The containment has the shape of a cylindrical, oftenmetal-lined concrete wall which is terminated at the top by a ceiling 3and at the bottom by a bottom 4. An internal partition wall 5 near thereactor 2 delimits a ring-formed space, the bottom part of which forms apool 6 for water. The water level is schematically indicated at 7. Theinternal partition wall 5 is terminated at the top by a circumferingceiling part 8. In the space above said ceiling part 8, a plurality ofline pipes 9 are mounted for feeding steam from the reactor 2, which inpractice may consist of a boiling reactor.

In the event steam would leak out from the line pipes 9, e.g. as aconsequence of unforeseen breakdowns, a particular steam blowingassembly is arranged in the containment, which assembly consists of aplurality of tubes 10, the upper and lower ends of which are open andwhich hang down from the ceiling part 8, more precisely immersed in thewater of the pool 6. In practice, the water depth in the pool may attain6-8 meters, the tubes 10 having a length of 12-15 meters so that thelower ends thereof are located close to the bottom of the pool. Inpractice, the number of tubes 10 in the assembly may amount to 30-90,each individual tube having a diameter of 50-60 cm.

In the event steam would come out in the top part of the containment, itmay be blown down through the tubes 10 so as to be condensed as fast aspossible in the water in the pool. Tests made have shown that steamwhich in this way rushes down through the individual tube under pressureapplies considerable mechanical stresses to the tube as a consequence ofexplosion-like shocks arising when the steam leaves the lower opening ofthe tube, at the same time as the water in the pool rises or heaves.During a certain phase, steam may be brought out in the pool and to acertain extent condense therein, but also to a certain extent cause thewater to rise, the water pressure in the bottom part of the poolincreasing, more precisely to a level at which the water pressureincreases above the steam pressure, water in a second phase beginning torise up in the blow down tubes while lowering the water level in thepool. Then implosion-like shock phenomena arise in the area of thelower, open ends of the tubes. After this, the phases may vary in such away that steam in one moment rushes down through the tubes and out inthe pool water and pool water, in another moment, rises up in the tubes.By the fact that the blow down tubes are often fastened only at theupper ends thereof, more precisely in the ceiling part 8, theabove-mentioned shock stresses on the lower, free-hanging ends of thetubes imply a security risk inasmuch as the tubes may be damaged or evencome loose under extreme circumstances.

In order to counteract this risk, tests have been made to strap thelower parts of the tubes by means of wires and the like. However, wireswhich are mounted in the pool are exposed to very large stresses whenthe water heaves, and therefore this solution is not reliable.Furthermore, tests have been made to modify the shape of the lower endsof the blow down tubes so that shock phenomena are mitigated. Amongother things, the lower end of the tube has been obliquely cut. Then,however, considerable transverse loads on the lower part of the tubesarise in connection with the shocks. Also, the lower end of the tube hasbeen made with a cross-section-wise semi-circular corbelling aroundwhich the steam and the water respectively may move more smoothly thanaround a sharp tube wall edge. However, nor these tests have beensuccessful, although a certain improving effect has been noted.

OBJECTS AND FEATURES OF THE INVENTION

The present invention aims at obviating the above-mentioneddisadvantages of previously known steam blowing assemblies and atcreating an improved assembly. Therefore, a primary object of theinvention is to provide a blow down tube meant for such assemblies, inwhich shock phenomena are reduced in connection the feeding of steamdown into the pool water as well as in connection with the rise ofpressurized water into the tube. A further object is to attain the setobject by means of, from a manufacturing point of view, technicallysimple and, accordingly, cheap means.

According to the invention, at least the primary object is attained bythe features defined in the characterising clause of claim 1. Preferredembodiments of the invention are furthermore defined in the dependentclaims.

BRIEF DESCRIPTION OF THE APPENDED DRAWING

In the drawing:

FIG. 1 is the above-mentioned perspective view which shows the generalconstruction of a nuclear power plant of known type,

FIG. 2 is an enlarged, partly cut view of the lower part of a blow downtube according to the invention,

FIG. 3 is a perspective view showing the very lowest portion of thetube,

FIG. 4 is a cross-section A--A in FIG. 2, and

FIG. 5 is a detailed side view showing an alternative embodiment, knownper se, of the bottom opening of the tube.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The blow down tube, designated 10 in its entirety according to FIG. 2,is in the form of a cylindrical plate wall 11, which at the bottom opensin a bottom opening 12. A number of through holes are recessed in thetube wall 11, each one of which being delimited by upper and lower edges13 and 14 respectively. At least one of said edges is part of a portionbent in relation to the tube wall as a whole. In the shown example, eachsuch bent portion 15 is formed in association with the lower hole edge14 and extends inwards from the tube wall as such, as may be clearlyseen in FIG. 3 and 4. In practice, the bent portion 15 may be madeintegrally with the tubewall as such, i.e. by plastic deformationthereof, though it is also conceivable to weld separate pieces of platein openings in the tube wall.

In the shown, preferred embodiment, the proper hole is in the form of aslot inasmuch as said upper and lower edges 13, 14 are situatedapproximately in one and the same horizontal plane, i.e. without anymarked level difference between the edges. In FIG. 4 is shown how thehole or the slot has an arc length along the tube wall corresponding toan arc angle α of approximately 30°. In practice, said arc angle shouldbe within the range of 15-60°, suitably 30-45°.

According to the shown, preferred embodiment, the slot holes arearranged in several sets separated along the tube, each one of whichcomprising at least two, suitably identical holes which are mutuallydiametrically opposite. Thus, two pairs of lower slot holes 16, 16';16", 16" on a level below a second set of two holes 17, 17' are shownwith a level difference corresponding to about the diameter of the tube.A third set of slot holes is shown at 18. The pairs of holes in adjacentsets of holes along the tube are advantageously rotationally displacedrelative to each other, more precisely in a 90° angle in the example. Inthe shown preferred embodiment, the lowest set of holes comprises fourequidistantly separated holes, while the sets of holes being abovecomprise only one pair of holes 17, 17'. The lowest set of holes isadvantageously situated close to the bottom opening 12, e.g. at adistance corresponding to about half of the diameter of the tube.

When a set of holes comprises four holes, said holes may be rotationallydisplaced in a 45° angle relative to the holes in adjacent sets(although the lower holes 16, 16'; 16", 16"' in the example are showndisplaced 90° in relation to the holes 17, 17' respectively in line withsaid holes).

When steam rushes down through the tube under increased pressure, e.g.within the range of 1-4 bar, the lower edge 14 in connection with eachindividual slot hole acts as a steam flow cutting edge which separates apart flow from the main steam flow and brings out said part flow to thesurrounding water in the pool before the main flow reaches the bottomopening 12 of the tube. By the fact that the steam in this way isdivided into a number of smaller part flows, the powerful pressure shockarising when one single main flow is to pass out in the water throughthe lower bottom opening, is mitigated or reduced. Therefore, a largerquantity of steam may, during a given unit of time, be brought out intothe water in a smoother way than when one single main flow is to be fedout. When the surrounding water in the pool in a second phase tends torise in the blow out tube as a consequence of increased water pressureand decreased steam pressure in the tube, the water may flow into theblow down tube not only through the central bottom opening 12, but alsothrough the described slot holes. In this case, the water flows inthrough the slot holes in the shape of jets and reduces, in addition,process-disturbing contraction and separation in the vicinity of thebottom opening.

In FIG. 5 it is indicated how a tube made with a slot hole according tothe invention may be supplemented by a circumfering, cross-section-wisearc-shaped flange 19 of a type known per se. Said flange contributes tofacilitate the flows of steam and water respectively through the bottomopening.

The invention is not solely restricted to the embodiment described andshown in the drawing. Thus, it is conceivable to form a bent portion inconnection with the upper edge 13 delimiting the individual slot hole,said portion bending outwards from the tube wall. Such a portion bendingoutwards may either act alone or in combination with a portion bendinginwards in connection with the lower hole edge 14 as has been shown inthe drawing.

I claim:
 1. Steam blowing assembly for nuclear power plants, comprisinga plurality of tubes (10), open at the top and at the bottom, hanging inconnection with a reactor (2), the lower ends of which tubes areimmersed in a water pool (6) and the purpose of which is to permit theblow down of steam into the water of the pool with the purpose ofcondensing the same so as to counteract the build-up of damaging steamoverpressure in the containment of the reactor, characterized in that anumber of through holes (16, 17, 18) are recessed in the wall (11) ofthe individual blow down tube (10), each one of said holes beingdelimited by upper and lower edges (13, 14) of which at least one (14)constitutes part of a portion (15) bent in relation to the tube wall(14), the lower hole edge (14) forming a steam flow cutting edge which,when the steam rushes down through the tube, divides a part flow fromthe main steam flow and brings said part flow out into the surroundingwater of the pool before the main flow reaches the bottom opening (12)of the tube, and said hole permitting feed of water to the tube in aphase when the water strives to flow up in said hole.
 2. Steam blowingassembly according to claim 1, characterized in that the bent tube wallportion (15) is made in connection with the lower hole edge (14) andextends inwards from the tube wall (11) as such.
 3. Steam blowingassembly according to claim 1, characterized in that said bent portion(15) is made integrally with the tube wall (11) as such, more preciselythrough plastic deformation thereof.
 4. Steam blowing assembly accordingto claim 1 characterized in that the hole is in the shape of a slotinasmuch as said upper and lower edges (13, 14) are situatedapproximately in one and the same horizontal plane, i.e. without anymarked level difference between the edges.
 5. Steam blowing assemblyaccording to claim 1, characterized in that the hole or slot has an arclength along the tube wall corresponding to an arc angle within therange of 15-60°, suitably 30-45°.
 6. Steam blowing assembly according toclaim 1, characterized in that the holes are arranged in two or moresets separated axially along the tube, each one of which comprising two,suitably identical holes (16, 16', 17, 17'), which are mutuallydiametrically opposite.
 7. Steam blowing assembly according to claim 6,characterized in that the pairs of holes in adjacent sets of holes alongthe tube are rotationally displaced, e.g. 90°, relative to each other.8. Steam blowing assembly according to claim 2, characterized in thatsaid bent portion (15) is made integrally with the tube wall (11) assuch, more precisely through plastic deformation thereof.
 9. Steamblowing assembly according to claim 2, characterized in that the hole isin the shape of a slot inasmuch as said upper and lower edges (13, 14)are situated approximately in one and the same horizontal plane, i.e.without any marked level difference between the edges.
 10. Steam blowingassembly according to claim 3, characterized in that the hole is in theshape of a slot inasmuch as said upper and lower edges (13, 14) aresituated approximately in one and the same horizontal plane, i.e.without any marked level difference between the edges.
 11. Steam blowingassembly according to claim 8, characterized in that the hole is in theshape of a slot inasmuch as said upper and lower edges (13, 14) aresituated approximately in one and the same horizontal plane, i.e.without any marked level difference between the edges.
 12. Steam blowingassembly according to claim 2, characterized in that the hole or slothas an arc length along the tube wall corresponding to an arc anglewithin the range of 15-60°, suitably 30-45°.
 13. Steam blowing assemblyaccording to claim 3, characterized in that the hole or slot has an arclength along the tube wall corresponding to an arc angle within therange of 15-60°, suitably 30-45°.
 14. Steam blowing assembly accordingto claim 4, characterized in that the hole or slot has an arc lengthalong the tube wall corresponding to an arc angle within the range of15-60°, suitably 30-45°.
 15. Steam blowing assembly according to claim2, characterized in that the holes are arranged in two or more setsseparated axially along the tube, each one of which comprising two,suitably identical holes (16, 16', 17, 17'), which are mutuallydiametrically opposite.
 16. Steam blowing assembly according to claim 3,characterized in that the holes are arranged in two or more setsseparated axially along the tube, each one of which comprising two,suitably identical holes (16, 16', 17, 17'), which are mutuallydiametrically opposite.
 17. Steam blowing assembly according to claim 4,characterized in that the holes are arranged in two or more setsseparated axially along the tube, each one of which comprising two,suitably identical holes (16, 16', 17, 17'), which are mutuallydiametrically opposite.
 18. Steam blowing assembly according to claim 5,characterized in that the holes are arranged in two or more setsseparated axially along the tube, each one of which comprising two,suitably identical holes (16, 16', 17, 17'), which are mutuallydiametrically opposite.
 19. Steam blowing assembly according to claim15, characterized in that the pairs of holes in adjacent sets of holesalong the tube are rotationally displaced, e.g. 90°, relative to eachother.
 20. Steam blowing assembly according to claim 16, characterizedin that the pairs of holes in adjacent sets of holes along the tube arerotationally displaced, e.g. 90°, relative to each other.